U.S. patent application number 11/138468 was filed with the patent office on 2005-12-22 for fuel cell aging method and apparatus.
This patent application is currently assigned to HITACHI CABLE, LTD.. Invention is credited to Sasaoka, Takaaki, Washima, Mineo.
Application Number | 20050282049 11/138468 |
Document ID | / |
Family ID | 35480963 |
Filed Date | 2005-12-22 |
United States Patent
Application |
20050282049 |
Kind Code |
A1 |
Sasaoka, Takaaki ; et
al. |
December 22, 2005 |
Fuel cell aging method and apparatus
Abstract
An aging apparatus is provided with a DMFC having anode and
cathode electrodes; anode-side and cathode-side separators for
feeding the anode and cathode electrodes with pure water or a
solution and an oxygen-containing gas, respectively; a
voltage-applying means for forcing current to flow between the
electrodes in the same direction as a direction of current flow
during power generation of the fuel cell; and a control means for
controlling the voltage-applying means.
Inventors: |
Sasaoka, Takaaki;
(Tsuchiura, JP) ; Washima, Mineo; (Tsuchiura,
JP) |
Correspondence
Address: |
FOLEY AND LARDNER LLP
SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
HITACHI CABLE, LTD.
|
Family ID: |
35480963 |
Appl. No.: |
11/138468 |
Filed: |
May 27, 2005 |
Current U.S.
Class: |
429/431 ;
429/442; 429/483; 429/506 |
Current CPC
Class: |
H01M 8/04302 20160201;
Y02E 60/523 20130101; H01M 8/04223 20130101; H01M 8/04186 20130101;
H01M 8/1011 20130101; Y02P 70/50 20151101; Y02E 60/50 20130101;
Y02P 70/56 20151101; H01M 8/04225 20160201 |
Class at
Publication: |
429/013 ;
429/012 |
International
Class: |
H01M 008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 21, 2004 |
JP |
2004-183070 |
Feb 10, 2005 |
JP |
2005-035288 |
Claims
What is claimed is:
1. A fuel cell aging method, comprising the steps of: feeding an
anode electrode of the fuel cell with pure water or a solution;
feeding a cathode electrode of the fuel cell with an
oxygen-containing gas; and forcing current to flow between the
electrodes in the same direction as a direction of current flow
during power generation of the fuel cell.
2. The fuel cell aging method according to claim 1, wherein: the
step of forcing current to flow is performed using a DC (direct
current) power supply.
3. The fuel cell aging method according to claim 2, wherein: the
step of forcing current to flow is performed with a current density
of 150-3000 mA/cm.sup.2.
4. The fuel cell aging method according to claim 1, wherein: the
step of forcing current to flow is performed using an AC
(alternating current) power supply.
5. The fuel cell aging method according to claim 1, wherein: the
step of forcing current to flow is performed until before the MEA
(membrane electrode assembly) temperature of the fuel cell reaches
100.degree. C., or until before the maximum applied voltage per
cell of the fuel cell reaches 3 V.
6. The fuel cell aging method according to claim 1, wherein: the
oxygen-containing gas is pure oxygen, air, or a nitrogen gas
containing 0.001-1% of oxygen.
7. The fuel cell aging method according to claim 1, wherein: the
fuel cell is a DMFC (direct methanol fuel cell).
8. A fuel cell aging apparatus, comprising: a fuel cell having
anode and cathode electrodes and requiring aging; aging medium feed
means for feeding the anode and cathode electrodes with pure water
ora solution and an oxygen-containing gas, respectively; a
voltage-applying means for forcing current to flow between the
electrodes in the same direction as that of current flow during
power generation of the fuel cell; and a control means for
controlling the aging medium feed means and the voltage-applying
means.
Description
[0001] The present application is based on Japanese patent
application Nos. 2004-183070 and 2005-035288, the entire contents
of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a fuel cell aging method
and apparatus, and particularly, an aging method and apparatus for
direct methanol fuel cells used in mobile and portable power
supplies, electric automobile power supplies, home cogeneration
systems, etc.
[0004] 2. Description of the Related Art
[0005] From the point of view of global environmental protection,
and the like, expectations for fuel cells have recently been
rapidly raised. Fuel cells are generally classified according to
kinds of electrolytes used into five kinds of solid oxide fuel
cells (SOFCs), molten carbonate fuel cells (MCFCs), alkaline fuel
cells (AFCs), phosphoric acid fuel cells (PAFCs), polymer
electrolyte fuel cells (PEFCs), etc.
[0006] Among others, polymer electrolyte fuel cells (hereinafter,
"PEFCs") with a polymer electrolyte membrane sandwiched between two
electrodes, in which these members are further sandwiched between
separators, are remarkable because of their compact structure,
excellent power generation efficiency and relatively
low-temperature operation, thereby having a wide range of
applications.
[0007] Also, among PEFCs, particularly, direct methanol fuel cells
(hereinafter, "DMFCs") are recently remarkable, which, instead of a
hydrogen gas, uses a methanol solution directly as fuel. DMFCs
generate power by causing an electrochemical reaction of a fuel
containing methanol and water, and an oxygen-containing oxidizer
gas such as air. DMFCs have various application fields. For
instance, because they operate at room temperature and can be made
small and sealed, they can be used in pollution-free automobiles,
home power generation systems, mobile communication equipment,
medical equipment, etc.
[0008] A DMFC comprises, basically, as a unit cell (hereinafter,
"cell"), a stacked body having conductive separators stacked on
both sides of a membrane electrode assembly (hereinafter, "MEA").
The MEA consists of three layers in which an electrolyte membrane
comprising an ion exchange resin or like is sandwiched between a
pair of electrodes constituting anode and cathode electrodes. The
pair of electrodes each consists of an electrode catalyst layer in
contact with the electrolyte membrane, and an outer fuel or
oxidizer gas diffusion layer (dispersion layer) of the electrode
catalyst layer. The conductive separators are stacked so as to come
into contact with the diffusion layer (dispersion layer) of the
MEA, and are formed with manifold apertures which serve as passages
for a fuel or oxidizer gas to flow into the diffusion layer
(dispersion layer), separator temperature adjustment, waste
removal, etc. Such fuel cells generate power by causing an
electrochemical reaction, for example, when a mixture of methanol
and water is caused to flow through the manifold apertures in
contact with the diffusion layer (dispersion layer) of the anode
electrode, while an oxidizing gas such as oxygen, air, or the like
is caused to flow through the manifold apertures in contact with
the diffusion layer (dispersion layer) of the cathode
electrode.
[0009] In DMFCs, because its power generation characteristic is
significantly low and unstable immediately after fuel cell
assembling, about 3-40 hour-power generation at a higher
temperature (typically, about 60-80.degree. C.) than room
temperature is required as initial preconditioning interim
operation (hereinafter, "aging") after DMFC assembling. This allows
higher cell output than the power generation characteristic
immediately after the assembling.
[0010] The problems in this aging are that, firstly, the aging
requires lengthy power generation, which results in an increase in
cost in mass production of fuel cells, and secondly, there are
restricted aging conditions after the fuel cells are incorporated
into equipment.
[0011] As a method for reducing the aging time of a PEFC, there is
known an aging method described in Japanese patent application
laid-open No. 2003-217622, for example.
[0012] Japanese patent application laid-open No. 2003-217622
discloses a method for operating a fuel cell having a polymer
electrolyte membrane which exhibits fuel ion conductivity by
retaining water, wherein the utilization ratio of a consumed gas is
improved so as to cause flooding within the fuel cell during
preliminary operation of the fuel cell.
[0013] As described therein, flooding caused in accordance with
this configuration provides the polymer electrolyte membrane with
water to increase its water content so as to form three-layer
interfaces properly, whereby the preliminary operation is
facilitated, and its required time reduced.
[0014] By the fuel cell aging method of Japanese patent application
laid-open No. 2003-217622, however, the above-mentioned aging
problems cannot be obviated sufficiently.
SUMMARY OF THE INVENTION
[0015] Accordingly, it is an object of the present invention to
provide a fuel cell aging method and apparatus, which are easily
and conveniently capable of reducing aging time and manufacturing
cost, and of aging after the fuel cell is incorporated into
equipment.
[0016] To achieve the above object, the present invention provides
a fuel cell aging method, comprising the steps of: feeding an anode
electrode of a fuel cell with pure water or a solution; feeding a
cathode electrode of the fuel cell with an oxygen-containing gas;
and forcing current to flow between the electrodes in the same
direction as that of current flow during power generation of the
fuel cell.
[0017] The features in the preferred embodiments of the invention
are as follows:
[0018] (1) The above current forcing is performed preferably using
a DC (direct current) power supply.
[0019] (2) The above current forcing is performed preferably with a
current density of 150-3000 mA/cm.sup.2.
[0020] (3) The above current forcing is performed preferably using
an AC (alternating current) power supply.
[0021] (4) The above current forcing is performed preferably until
before the MEA temperature of the fuel cell reaches 100.degree. C.,
or until before the maximum applied voltage per cell of the fuel
cell reaches 3 V.
[0022] (5) The above oxygen-containing gas is preferably pure
oxygen, air, or a nitrogen gas containing 0.001-1% of oxygen.
[0023] (6) The above fuel cell is preferably a DMFC.
[0024] To achieve the above object, the present invention provides
a fuel cell aging apparatus, comprising: a fuel cell having anode
and cathode electrodes and requiring aging; aging medium feed means
for feeding the anode and cathode electrodes with pure water or a
solution and an oxygen-containing gas, respectively; a
voltage-applying means for forcing current to flow between the
electrodes in the same direction as that of current flow during
power generation of the fuel cell; and a control means for
controlling the aging medium feed means and the voltage-applying
means.
[0025] The features in the preferred embodiments of the invention
are the same as above (1)-(6).
[0026] The aging method and apparatus according to the invention
can realize making aging convenient, reducing aging time, relaxing
aging conditions, and thereby making fuel cells low-cost. It is
also easily capable of aging after fuel cells are incorporated into
equipment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The preferred embodiments according to the invention will be
explained below referring to the drawings, wherein:
[0028] FIG. 1 is a diagram illustrating a schematic configuration
of an aging apparatus according to an embodiment of the
invention;
[0029] FIG. 2 is a diagram showing current-voltage curves during
aging according to the invention;
[0030] FIG. 3 is a time chart during aging according to the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] FIG. 1 illustrates a schematic configuration of an aging
apparatus according to an embodiment of the invention. This aging
apparatus 10 schematically comprises a DMFC 1 as-assembled; a
voltage-applying means 11 as an electric field-applying means for
forcing current to flow by applying voltage to the DMFC 1; and a
control means 12 for controlling the voltage-applying means 11.
Further, the DMFC 1 may be integrated with or separated from the
voltage-applying means 11, etc. Also, although here is explained
the DMFC that has particularly remarkable effects, the invention is
not limited thereto, but may be applied to fuel cells requiring
aging. It is preferably applied to polymer electrolyte fuel cells,
and particularly to DMFCs.
[0032] The DMFC 1 may be a well-known DMFC, and that cell comprises
anode- and cathode-side separators 2A and 2B, anode and cathode
electrodes 3A and 3B; and an electrolyte membrane 4. The anode and
cathode electrodes 3A and 3B and electrolyte membrane 4 constitute
an MEA 5 which is sandwiched between the anode- and cathode-side
separators 2A and 2B. The DMFC 1 is generally used by connecting a
plurality of cells in series according to target electromotive
force.
[0033] The anode and cathode electrodes 3A and 3B each comprises a
support layer for feeding and diffusing (dispersing) a fuel or an
oxidizer gas, and a catalyst layer for oxidization or reduction
reactions to occur. In the anode electrode 3A, by an oxidization
reaction of methanol and water fed, hydrogen ions, electrons, and
carbon dioxide are produced, and the hydrogen ions produced are
conducted through the electrolyte membrane 4 to the cathode
electrode 3B, while the electrons produced are conducted through an
external circuit to the cathode electrode 3B. In the cathode
electrode 3B, water is produced by a reduction reaction of hydrogen
and oxygen ions.
[0034] For a polymer electrolyte membrane of the electrolyte
membrane 4, a thin membrane (thickness: the order of 50-100 .mu.m)
may be used having a perfluorocarbon sulfonic acid structure having
a sulfonic acid group as an ion exchange membrane, for example,
although not limited thereto, to allow fabricating a compact
cell.
[0035] The anode-side separator 2A is formed with a fuel feed
groove for feeding a fuel to the adjacent anode electrode 3A, while
the cathode-side separator 2B is formed with an oxidizer gas feed
groove for feeding an oxidizer gas to the adjacent cathode
electrode 3B, so that a fuel and oxidizer gas are fed along the
surfaces of the separators 2A and 2B, respectively.
[0036] For the separators 2A and 2B, there may be suitably used a
carbon separator, a carbon compound-molded separator with carbon
kneaded into resin, a metallic separator having on its surface a
corrosion-resistant layer formed of titanium, stainless steel or
noble metals, etc, although not limited thereto.
[0037] Based on a command from the control means 12, the
voltage-applying means 11 applies voltage to the DMFC 1 to thereby
force current to flow therein. Using a DC (direct current) power
supply is preferred, but an AC (alternating current) power supply
may be used. Also, the control means 12 is provided with a CPU,
etc., which controls aging, as described later.
[0038] An aging method according to an embodiment of the invention
will be explained next. As the aging method, its procedure is as
follows:
[0039] 1. An anode medium 6A is fed to the anode electrode 3A of
the DMFC 1, while a cathode medium 6B is fed to its cathode
electrode 3B. This feed method may be any method for feeding in a
natural flow even in the case of forced circulation.
[0040] 2. A DC power supply (voltage-applying means 11) is
prepared. The anode electrode 3A of the DMFC 1 is connected to a
positive terminal of the DC power supply output, while its cathode
electrode 3B is connected to a negative terminal of the DC power
supply output. Such connection allows forcing current to flow
through the MEA 5 in the same direction as that during normal power
generation. An AC power supply may be used.
[0041] 3. Using the DC power supply, current is forced to flow
through the DMFC 1. The current forcing conditions are as follows:
For current, the current density Je per electrode surface area of
the MEA 5 is within a range of 300-3000 mA/cm.sup.2; the voltage
between the terminals per cell of the DMFC 1 is 0.3-3 V; and the
current flow time is a few seconds to a few minutes. An AC power
supply of .+-.3000 mA/cm.sup.2 or less may be used in current
forcing. During current flowing, the anode is fed so as not to run
out of water content in the anode side. By repeating this plural
times, the DMFC 1 aging process is completed.
[0042] The aging method according to the embodiment of the
invention will be explained in more detail below.
[0043] (1) Anode Medium 6A
[0044] For the anode medium 6A, water or a methanol solution is
used. In actual DMFC power generation, since the methanol
concentration is used on the order of 0.1-10 mol/l, filling a
methanol solution of this concentration range is preferred. On the
other hand, even in the case of use of pure water, if after current
forcing for aging, it is replaced with a methanol solution for DMFC
power generation, pure water may be used in current forcing, but in
order to save labor and time in the replacement, it is preferred to
use a methanol solution which is typically used as the fuel. Also,
the essential point of the invention is considered to lie in the
electrolysis of water, and any solution in which the MEA 5 is not
damaged by the anode medium 6A may therefore be used without being
limited to water and a methanol solution. For instance, there may
be used an ethanol solution, isopropyl alcohol solution, etc. As
its feed method, there are a method for attaching a solution tank
to the anode electrode 3A, a method for feeding a solution by
forced circulation, and so on, without a particular limitation.
Apart from the electrolysis of water, the essential point of the
invention is also considered to be related to the synthesis of
water, and the effects of the invention are exhibited by a combined
action of both.
[0045] (2) Cathode Medium 6B
[0046] For the cathode medium 6B, an oxygen-containing gas such as
air (an oxygen-containing nitrogen gas) is fed. The oxygen content
may be as high as the concentration of a pure oxygen gas, or be as
low as a concentration of the order of 0.001-1%, without a
particular limitation. Any oxygen-containing gas may be used, and
the oxygen concentration and the kinds and concentrations of other
gases contained may be selected appropriately from the point of
view of convenience, economy, etc. As its feed method, there are a
feed method by a natural breathing DMFC structure in which the
cathode electrode 3B is left unattended in atmosphere, a method for
feeding a gas by forced circulation, and soon, without a particular
limitation.
[0047] (3) Current Density J.sub.e in Current Forcing
[0048] In actual DMFC power generation, power is generated
typically in a range of the order of 0-200 mA/cm.sup.2. Current
forcing requires a current flow of more than a load current density
assumed in actual DMFC power generation, and it is therefore
preferred to force constant current whose current density is within
a range of 150-3000 mA/cm.sup.2, more preferably, 250-2000
mA/cm.sup.2, still more preferably, 350-1500 mA/cm.sup.2, and most
preferably, 400-1400 mA/cm.sup.2. Too small a current density would
have no effect, while too large a current density would cause
thermal destruction to the MEA 5. At a current density of 2500
mA/cm.sub.2 or higher, in order to prevent thermal destruction of
the MEA 5, it is preferred to cool the cell, or reduce current flow
time (e.g. to a few seconds) in current forcing. From the point of
view of convenience, etc., it is preferred to force constant
current whose value is within the above range.
[0049] In the case of use of an AC power supply, it is preferred to
force current whose current density is within a range of .+-.3000
MA/cm.sup.2, more preferably, .+-.2000 MA/cm.sup.2, still more
preferably, .+-.1500 mA/cm.sup.2, and most preferably, .+-.1400
mA/cm.sup.2.
[0050] Even in case of too small a current density, the effects of
the invention can be enhanced by increasing current flow time or
the number of times of repeating current flow.
[0051] (4) Applied Voltage V in Current Forcing
[0052] During current flow, it is preferred to apply, as the
voltage between the electrodes per cell, a voltage of 0.3-3 V, more
preferably, 0.6-2.7 V, and still more preferably, 0.9-2.5 V. Too
small a voltage would cause no electrolysis, which therefore has
almost no aging effect. Too large a voltage can result in an
undesirable thermal or electrical damage.
[0053] (5) Current Flow Time t and the Number of Times of Current
Forcing in Current Forcing
[0054] The current flow time is preferably a few seconds to a few
minutes. Too short a current flow time would have no effects. Too
long a current flow time would undesirably result in an increase of
the temperature of the MEA 5 due to heat generation, and
consequently, in an electrochemical reaction (e.g. corrosion) of
the separators progressing. The proper current flow time is a
current flow time until before the temperature of the MEA 5 reaches
100.degree. C., or until before the maximum voltage per cell during
constant current forcing reaches 3 V, at which the current flow is
zero. It is preferred to repeat such operation 2-6 times, more
preferably, 3-5 times. Repeating current flow would make voltage at
the start of current flow lower than voltage at the preceding start
of current flow, but it is preferred to repeat current flow until
almost no voltage reduction is caused, and more preferably, until
before no voltage reduction is caused. In other words, it is
preferred to repeat current flow until the cell internal resistance
stabilizes. For example, if the voltage at the fourth start of
current flow is substantially the same as the voltage at the third
start of current flow, it is preferred to complete the aging
process at the third current forcing.
[0055] (6) The conditions for the applied voltage V, forced current
I (current density J.sub.e) and current flow time t in (3)-(5)
above are determined by the following steps of:
[0056] (a) measuring applied voltage V and forced current I of a
cell and monitoring V-I characteristics during aging;
[0057] (b) increasing current and voltage by a DC power supply
(FIG. 2);
[0058] (c) forcing constant current in a current-voltage range
below a point (around 10A in FIG. 2) at which dV/dI of the
current-voltage graph increases sharply with increasing current,
(hereinafter, "moderate range"). FIG. 2 shows current-voltage
curves during aging according to the invention, where a constant
current of approximately 10 A is forced to flow. It is also
possible to apply a current-voltage range exceeding the moderate
range, (hereinafter, "over current range")), by adjusting forced
current flow time or cooling.
[0059] (d) stopping the current flow when the temperature of the
MEA 5 reaches 70-100.degree. C., or when the maximum voltage per
cell during constant current forcing reaches 1-3 V. The current
flow time at which this is done is taken to be t.sub.1.
[0060] (e) repeating steps (c) and (d) 3-6 times, making sure that
the temperature of the MEA 5 is decreased below 50.degree. C., or
below a temperature immediately after current is caused to flow. In
order to reduce wait time, forced cooling may be performed. The
repeated current flow time is varied according to the temperature
of the MEA 5 at the repeated starts of current flow.
[0061] (7) Instead of (6) above where the conditions for the
applied voltage V, forced current I (current density J.sub.e) and
current flow time t are determined, current may be caused to flow
with the values of V, I, and t which satisfy the following
(1)-(3):
.DELTA.T<100 (1)
T.sub.2<100 (2)
q<100 (3)
.times.T=(V.sub.1+V.sub.2)/2.times.I.times.t/(C.sub.2.multidot..rho..sub.2-
.multidot.V.sub.a) (4)
q=V.times.I/S[W/cm.sup.2] (5),
[0062] where
[0063] .DELTA.T: an estimated value for a temperature rise due to
current flow [.degree. C.]
[0064] T.sub.1: the temperature of the MEA 5 before current flow
[.degree. C.]
[0065] T.sub.2: the temperature of the MEA 5 immediately after end
of current flow [.degree. C.]
[0066] V.sub.1: the applied voltage immediately after start of
constant current flow
[0067] V.sub.2: the applied voltage immediately before end of
constant current flow
[0068] C.sub.2: the specific heat of the anode injection solution
[J/(g.multidot.K)]
[0069] .rho..sub.2: the density of the anode injection solution
[g/cm.sup.3]
[0070] v.sub.a: the anode injection solution amount per one MEA
[cm.sup.3/sec]
[0071] S: the electrode surface area of the MEA [cm.sup.2]
[0072] When the temperature T.sub.1 of the MEA 5 is on the order of
room temperature (25-30.degree. C.), it is preferred that
.DELTA.T<60-70.
[0073] The reason for .DELTA.T [.degree. C.]<100 is because, at
T.sub.1>0, the maximum temperature of the MEA 5 does not exceed
100.degree. C. The reason for q [W/cm.sup.2]<100 is because q
[W/cm.sup.2]>100 would cause an interface transition from
nuclear boiling to membrane boiling, which would result in poor
heat dissipation, and further produce a boiling membrane in an
anode interface which would undesirably inhibit electrolysis of
water.
[0074] (8) dV/dI Characteristics During Current Forcing
[0075] It is preferred to cause large current to flow by applying
as small a voltage as possible. To this end, it is preferred to
cause constant current flow whose value is below the moderate
range.
[0076] FIG. 3 is a time chart during aging according to the
invention, and shows the case of three-time constant current
(current density J.sub.e [mA/cm.sup.2]) forcing in the above
conditions.
[0077] The above aging method allows quick aging, which serves
conveniently even as ancillary facilities, compared to conventional
methods. Because of the specified current flow, aging is also made
possible after the DMFC 1 is incorporated into equipment.
EMBODIMENT
[0078] (1) Making a Fuel Cell for Experiment
[0079] As separators for a DMFC, metal cladding sheet materials
having corrosion resistance and surface conductivity is fabricated.
Using a composite metallic member of Ti/SUS/Ti in which stainless
steel (SUS 304) is used as the core metal and metal titanium is
used as the cladding metal, surface treatment is performed on this
member for having combined conductivity and corrosion resistance,
by the method disclosed in Japanese patent application laid-open
No. 2004-158437. Using this metallic member and an MEA (Nafion
(registered trademark) used as the electrolyte membrane), a cell
with an electrode surface area S=8.4 cm.sup.2) is assembled.
[0080] (2) Current Forcing Experiment 1
[0081] Using a cell assembled, this current forcing experiment is
performed. In this experiment, the anode injection solution amount
is taken to be v.sub.a=1 [cm.sup.3/sec]. Also, the specific heat of
the anode injection solution is taken to be the value of water as a
representative value, C.sub.2=4.2 [J/(g.multidot.K)] and
.rho..sub.2=1.0 [g/cm.sup.3]. Accordingly, since I=electrode
surface area S.times.forced current density J.sub.e, substituting
into equation (4) above, it follows that
.DELTA.T=(V.sub.1+V.sub.2)/J.sub.e.times.t.
[0082] A DC power supply is prepared. The anode electrode of the
DMFC is connected to a positive terminal of the DC power supply
output, while its cathode electrode is connected to a negative
terminal of the DC power supply output. Current is forced to flow
in the same direction as that during normal power generation, so
that the current density is constant. In each sample, the number of
times of current forcing is three. Using pure water (sample 8) or a
methanol solution (0.1 mol/l, 1.0 mol/l, 3.0 mol/l, 8.0 mol/l, 10.0
mol/l) (samples 1-7, 9-16) as the anode feed solution and air
(samples 1-12), a nitrogen gas containing 0.001-1% of oxygen
(samples 13-16) or pure oxygen (oxygen left after removing
inevitable impurities) (sample 17) as the cathode feed gas, the
experiment is implemented by forcing each of them to circulate.
[0083] For each sample cell, current forcing is performed in
various conditions and subsequently, DMFC power generation
characteristic assessment is performed at room temperature
25.degree. C., using air (samples 1-16) and pure oxygen (sample 17)
as the cathode feed gas, and taking the cathode feed gas
utilization ratio as 10%. The assessment results are shown in Table
1. Table 1 shows forced current conditions, and maximum outputs
obtained for the DMFC power generation characteristic. The maximum
outputs are converted into values per MEA electrode surface area.
Table 1 also shows voltage V.sub.1 immediately after the start of
the constant current forcing and voltage V.sub.2 immediately before
the end of the current flow. In Table 1, .DELTA.T is the calculated
values, and T.sub.1 and T.sub.2 are the measured values.
[0084] Next, for the following cells as comparison examples, DMFC
power generation characteristic assessment is performed in the same
manner as the above embodiments: cells on which no aging is
performed (Samples X.sub.1 and X.sub.2 use air and pure oxygen,
respectively, as the cathode feed gas in the DMFC power generation
characteristic assessment); and cells on which DMFC power
generation is performed at 60.degree. C. for 8 hrs as aging
(Samples Y.sub.1-Y.sub.4 and Y.sub.5 use air and pure oxygen,
respectively, as the cathode feed gas in the aging and DMFC power
generation characteristic assessment). The assessment results are
shown in Table 2.
1TABLE 1 Assessment results of the prototype fuel cell (EMBODIMENT)
Forced current-conducting conditions DMFC power generation current
applied applied current .DELTA.T (.degree. C.) characteristic (25
.degree. C .) Sample anode injection density J.sub.o voltage
V.sub.1 voltage V.sub.2 flow time t T1 (.degree. C.) maximum output
No. solution v.sub.o (mol/l) (mA/cm.sup.2) (V) (V) (sec) T2
(.degree. C.) (mW/cm.sup.2) 1 methanol solution 150 0.6 0.7 200
.DELTA.T = 39 10 (1.0) T1 = 30 T2 = 80 2 methanol solution 300 0.9
1.1 58 .DELTA.T = 35 20 (1.0) T1 = 30 T2 = 70 3 methanol solution
450 1.2 1.4 30 .DELTA.T = 35 25 (1.0) T1 = 30 T2 = 70 4 methanol
solution 750 1.7 2.0 13 .DELTA.T = 36 25 (1.0) T1 = 30 T2 = 71 5
methanol solution 1050 2.0 3.0 7 .DELTA.T = 37 25 (1.0) T1 = 30 T2
= 72 6 methanol solution 450 1.2 2.0 40 .DELTA.T = 58 25 (1.0) T1 =
30 T2 = 95 7 methanol solution 450 1.2 2.4 45 .DELTA.T = 73 15
(1.0) T1 = 30 T2 = 105 8 pure water 450 1.8 2.2 30 .DELTA.T = 54 25
T1 = 30 T2 = 85 9 methanol solution 450 1.2 1.4 30 .DELTA.T = 35 25
(0.1) T1 = 30 T2 = 70 10 methanol solution 450 1.2 1.4 30 .DELTA.T
= 35 18 (10.0) T1 = 30 T2 = 70 11 methanol solution 450 1.2 1.4 30
.DELTA.T = 35 25 (3.0) T1 = 30 T2 = 70 12 methanol solution 450 1.2
1.4 30 .DELTA.T = 35 25 (8.0) T1 = 30 T2 = 70 13 methanol solution
450 1.2 1.4 30 .DELTA.T = 35 25 (1.0) *1 T1 = 30 T2 = 70 14
methanol solution 450 1.2 1.4 30 .DELTA.T = 35 25 (1.0) *2 T1 = 30
T2 = 70 15 methanol solution 450 1.2 1.4 30 .DELTA.T = 35 25 (1.0)
*3 T1 = 30 T2 = 70 16 methanol solution 450 1.2 1.4 30 .DELTA.T =
35 25 (1.0) *4 T1 = 30 T2 = 70 17 methanol solution 450 1.2 1.4 30
.DELTA.T = 35 30 (1.0) *5 T1 = 30 T2 = 70 *1: Sample 13 uses a
mixture of (1% oxygen + remaining nitrogen) as the cathod feed gas
in current forcing (aging), and air as the cathod feed gas in
assessment of the DMFC power generation characteristic. *2: Sample
14 uses a mixture of (0.1% + oxygen + remaining nitrogen) as the
cathod feed gas in current forcing (aging), and air as the cathod
feed gas in assessment of the DMFC power generation characteristic.
*3: Sample 15 uses a mixture of (0.01% oxygen + remaining nitrogen)
as the cathod feed gas in current forcing (aging), and air as the
cathod feed gas in assessment of the DMFC power generation
characteristic. *4: Sample 16 uses a mixture of (0.001% oxygen +
remaining nitrogen) as the cathod feed gas in current forcing
(aging), and air as the cathod feed gas in assessment of the DMFC
power generation characteristic. *5: Sample 17 uses pure oxygen as
the cathod feed gas in current forcing (aging), and assessment of
the DMFC power generation characteristic.
[0085]
2TABLE 2 Assessment results of the prototype fuel cell (COMPARISON
EXAMPLE) anode injection DMFC power generation Sample solution
characteristic No. (mol/l) Aging conditions (25.degree. C.) X.sub.1
methanol absence of aging 8 solution(1.0) X.sub.2 methanol
12*.sup.6 solution(1.0) Y.sub.1 methanol 8 hour power 25
solution(1.0) generation at 60 degree C. using air as the cathod
feed gas Y.sub.2 methanol 18 solution(10.0) Y.sub.3 methanol 22
solution(3.0) Y.sub.4 methanol 20 solution(8.0) Y.sub.5 methanol 8
hour power 30*.sup.6 solution(1.0) generation at 60 degree C. using
pure oxygen as the cathod feed gas *.sup.6Pure oxygen is used as
the cathod feed gas in assessment of the DMFC power generation
characteristic.
[0086] For samples 1-5, this current forcing experiment is
performed in the condition of .DELTA.T=30-40. For sample 1, no
distinct hydrogen generation can be detected during the current
forcing, whereas, for samples 2-5, hydrogen generation is
detected.
[0087] Samples 2-5 exhibit substantially the same DMFC power
generation characteristic as that of comparison sample Y.sub.1, so
the current forcing has the aging effect on samples 2-5. Although
sample 1 exhibits the lower DMFC power generation characteristic
than that of comparison sample Y.sub.1, it exhibits the higher DMFC
power generation characteristic than that of comparison sample
X.sub.1 on which no aging is performed, and the current forcing is
therefore considered to have the aging effect on sample 1 as well.
In other words, for sample 1, the current forcing conditions are
considered to indicate a lower limit at which the aging effect
develops.
[0088] Also, for sample 6, the current forcing experiment is
performed in the condition of .DELTA.T=58. Sample 6 exhibits the
same DMFC power generation characteristic as that of comparison
sample Y.sub.1, so the current forcing has the aging effect on
sample 6. On the other hand, for sample 7, the current forcing
experiment is performed in the condition of .DELTA.T=73
(T.sub.2-105). Sample 7 exhibits the lower DMFC power generation
characteristic than that of comparison sample Y.sub.1. From this,
in the current forcing conditions of .DELTA.T>70 and T.sub.1=30,
i.e., T.sub.2>100, the MEA is considered to degrade due to water
or solution boiling, which results in a decrease of the cell
function. In other words, the conditions in which the water
(solution) boils inside the cell are considered to indicate an
upper limit of effective aging.
[0089] In addition to the cases of 1.0 mol/l-methanol solutions in
samples 1-7, as shown in the results of samples 8, 9 and 10, when
the anode injection solution is pure water, 0.1 and 10.0
mol/l-methanol solutions, the DMFC power generation characteristic
after current forcing (aging) exhibits the suitable values (the
same value as that of comparison sample Y.sub.2, in the case of the
10.0 mol/l-methanol solution), so the current forcing has the aging
effect.
[0090] Also, both when the anode injection solution is 3 and 8
mol/l-methanol solutions (samples 11 and 12), and when the cathode
feed gas is a nitrogen gas containing 0.001-1% of oxygen (samples
13-16), the DMFC power generation characteristic after current
forcing is the same as or higher than those of comparison samples
Y.sub.3 and Y.sub.4, so the current forcing has the aging
effect.
[0091] Further, when the cathode feed gas is pure oxygen (sample
17), the DMFC power generation characteristic after current forcing
is higher than that of comparison sample X.sub.2 on which no aging
is performed, and exhibits the same value as that of comparison
sample Y.sub.5, so the current forcing has the aging effect.
[0092] From the above implementation results, current forcing is
considered to have the aging effect in various combinations of an
anode injection solution range at least from pure water to 10.0
mol/l-methanol solutions, and a cathode feed gas range at least
from a nitrogen gas containing 0.001% of oxygen to pure oxygen.
[0093] (3) Current Forcing Experiment 2
[0094] Using DC and AC power supplies, this current forcing
experiment is performed. First, the anode electrode of the DMFC is
connected to a positive terminal of the DC power supply output,
while its cathode electrode is connected to a negative terminal of
the DC power supply output. Current is forced to flow in the same
direction as that during normal power generation (current forcing
order: 1). Subsequently, by reverse connection, current is forced
to flow in the reverse direction to a direction during normal power
generation (current forcing order: 2). Next, by causing the
connection to return to the first connection state, current forcing
is performed (current forcing order: 3), and then by replacing the
DC power supply to connect the AC power supply, current forcing is
performed (current forcing order: 4). Further, for one cell (sample
17), by changing the conditions in order shown in Table 3, current
forcing is performed continuously (current forcing order: 5-8). The
number of times of current forcing is three in only the first
conditions (current forcing order: 1; +450 mA/cm.sup.2; 30 sec),
and one in the subsequent current forcing conditions. Using a
methanol solution (1.0 mol/l) as the anode feed solution and air as
the cathode feed gas, the experiment is implemented by forcing each
of them to circulate. Subsequently, the DMFC power generation
characteristic is assessed in the same manner as in "current
forcing experiment 1". The assessment results are shown in Table
3.
3TABLE 3 Assessment results of the prototype fuel cell (EMBODIMENT)
Current-forcing conditions DMFC power generation Current- current
current characteristic (25.degree. C.) forcing density J.sub.e flow
time t maximum output order (mA/cm.sup.2) (sec) (mW/cm.sup.2) 1
+450 30 25 2 -450 30 10 3 +450 30 30 4 .+-.400 30 30 5 -750 13 5 6
+750 13 30 7 +2000 3 30 8 +3000*.sup.7 20 25 +: the same direction
as a direction in normal power generation -: the reverse direction
to a direction in normal power generation .+-.: AC current flow
*.sup.7Cooling is being performed during current forcing.
[0095] From Table 3, itis seen that current forcing in the reverse
direction to a direction during normal power generation (current
forcing order: 2 and 5) exhibits no effect of the invention, and
that current forcing by the AC power supply (current forcing order:
4) exhibits the effect of the invention since current forcing is
also performed in the same direction as a direction during normal
power generation. It is also seen that even the current density in
the overcurrent range (current forcing order: 7 and 8) is made
applicable by current-flow time adjustment or cell cooling during
current forcing.
[0096] Although the invention has been described with respect to
the specific embodiments for complete and clear disclosure, the
appended claims are not to be thus limited but are to be construed
as embodying all modifications and alternative constructions that
may occur to one skilled in the art which fairly fall within the
basic teaching herein set forth.
* * * * *